An important problem in electronics manufacturing is automatic verification of connection between a component or a component lead and a trace on a printed circuit board. The problem is especially important for surface mount large scale integrated circuits with high lead counts and high lead density, where the soldering process often leaves some leads unconnected. It is also an important problem for large scale integrated circuits placed in sockets where leads may be bent or sockets may not make contact.
In general, electrical verification of a connection requires stimulating a device under test and detecting an electrical response which indicates the presence of a viable connection. In general, the problem is complicated by parallel electrical paths. It is difficult to isolate a single lead if a stimulus induces electrical responses in multiple parallel paths.
An example prior art solution is disclosed in U.S. Pat. No. 5,111,137. That patent discloses a system in which intact electrical paths are detected by measuring a change in semiconductor leakage current resulting from irradiating a semiconductor with ionizing radiation such as x-rays. The primary stimulus is a voltage applied to a trace on a printed circuit board. The voltage results in a leakage current through diodes in the attached semiconductor devices. A secondary stimulus (ionizing radiation) then induces a change in that leakage current for one device. An individual semiconductor device is irradiated and any change in current is detected. A change in current during irradiation indicates that a semiconductor device is present, that there is an intact connection between the semiconductor device and the trace on the printed circuit board, that bond wires to the integrated circuit are intact, and that there is an intact connection from the integrated circuit to ground.
In general, ionizing radiation sources such as x-rays create safety concerns and require extensive measures to protect human operators. In addition, x-rays may damage some crystals, some thin-oxide MOS-FETs or other particularly sensitive pans. There is a need for a simpler, safer system to induce a change in current.